As is well known, in the industry of paper a variety of types of machines and of processes exist for making paper tissues, paper towels and the like by producing stacks of interfolded sheets of a certain height.
They are obtained by folding the sheets in an “interfolded” way, namely folding into panels by overlapping at the same time a panel of a previous sheet with a panel of a following sheet. In this way, when pulling a sheet from the stack, at the moment of the use also a panel of the following sheet is pulled out, with consequent advantages for certain types of uses. Among the possible interfolding ways, stacks of L, Z or W interfolded sheets are known having 2, 3 and 4 panels, respectively.
Machines are known that use one or more webs of paper, coming from one or more reels, that are cut into sheets and then supplied offset with respect to one another on folding counter-rotating rollers.
More precisely, the webs are cut into sheets by means of cutting rollers that engage with respective blades. In the case of L or W interfolding, the webs are cut so that they form a sequence of offset sheets coming preferably from two different directions. Therefore, the sheets coming from both directions are supplied alternately to the folding rollers so that each sheet coming from a first direction overlaps a portion of a sheet coming from the second direction, and vice versa.
The sheets coming from either directions, in order to be overlapped in the above described way, adhere to the respective folding rollers by means of a vacuum-suction step or by means of a mechanical gripping. Therefore, the downstream portion of each sheet leaves its folding roller at the point of contact between the two rollers, then adhering to the other folding roller, to which the upstream portion of the previous sheet has adhered.
The method for Z interfolding is similar as above described, with the difference being that the overlap between two consecutive sheets occurs just after the cutting step and a sequence of overlapping and offset sheets come to the folding rollers from only one direction.
The interfolded stacking step is accomplished with the aid of folding elements, which in the case of rollers with mechanical folding means, consist of mechanical clamps incorporated in the roller. In case instead of folding rollers with suction system the folding elements consist of folding arms that have an oscillating motion about a pivot and that separate in turn from the respective roller the upstream portion of each sheet joined to the overlapped downstream portion of the following sheet. The folding arms are normally arranged in two rows and operate alternatively with the paired portions of sheets, which adhere to the first or to the second folding roller.
The folding rollers with suction folding means have a plurality of circumferential grooves, into which the ends of said folding arms go without blocking their rotation. At the passage of two overlapped portions of two consecutive sheets, that adhere to a roller and cover an end of the folding arms, the folding arms rotate so that their ends go out the grooves and push the two overlapped portions away from the folding roller, thus folding them onto previously interfolded sheets stacked below.
The folding rollers have a circumference that is normally multiple of the length of the sheets. Therefore, a sheet is added to the stack of interfolded sheets at each respective fraction of turn of the folding rollers.
Downstream from the rollers above described, along the direction of process, a plurality of means for separation is furthermore provided that enters the stack for all or part of its width. This way, a package is separated from a next one, and precisely the packages of a predetermined height are put in turn on a support table that eventually loads them on a conveyor and then moves back for receiving another pack.
As said above, in the interfolding machines the length of the interfolded sheets that form the stack of final product is due to the circumference of the cutting rollers and to the angular distance at which the cutting means are arranged. In other words, the length of cut is fixed and is measured univocally by the circumference of the rollers. In particular, both the folding rollers and the cutting rollers have a circumference multiple to the length of the sheets. For this reason each interfolding machine causes the production of interfolded sheets of a single length, whereby the production process is not flexible. For sheets of different length it is therefore necessary to provide different machines.